In the engraved steel die printing process, hereafter called intaglio printing process, a rotating engraved steel cylinder, carrying a pattern or image to be printed, and heated to a temperature of the order of 80° C., is supplied with ink by one or more template inking cylinders. Subsequent to the inking, any excess of ink on the plain surface of the printing cylinder is wiped off by a rotating wiping cylinder. The remaining ink in the engraving of the printing cylinder is transferred under pressure onto the substrate to be printed, which may be a paper or plastic material in sheet form, while the wiping cylinder is cleaned by a wiping solution. Other wiping techniques can also be used, such as paper wiping or tissue wiping (“calico”).
One of the distinguishing features of the intaglio printing process is that the film thickness of the ink transferred to the substrate can be varied from a few micrometers to several tens of micrometers by a correspondingly shaped printing plate. This ability to vary the film thickness is a most desirable feature of the intaglio printing process and can be used to produce embossing effects, i.e. to confer tactility to the printed document, as well as to produce shade variations using one and the same ink.
The pronounced relief of the intaglio printing accentuates the problem of “set-off”, which is the transfer of ink from one printed sheet to the back side of the next following printed sheet in a stack, or to the back of the endless sheet in a web. The factors influencing the “set-off” are determined by the printing ink formulation, the engraving depth and evenness, the printing conditions, the printing substrate, the number of stacked sheets per pile, the time between printing and handling of the piles and the way how the printed piles of paper are handled after printing.
The “set-off” caused by the residual tackiness of the printed ink, which adheres to the substrate surface of the back of the next sheet, is aggravated when pressure is applied to a pile of stacked printed sheets. Depending on its extent, “set-off” can irreversibly spoil the printed product affected by it. A classical method to avoid losses of printed good due to “set-off” is to interleave separation sheets between all printed sheets; this leads however to a slowing down of the printing process and also to a more expensive printing.
The problem of reducing set-off in oxidatively curing inks has been addressed in the art in several ways:
by using high molecular weight oxidatively curable binders,
by solvents with relatively low boiling point which would partially evaporate on the printing plate,
by waxes, forming a protective layer on the ink film,
by a high filler to binder ratio which would reduce the residual tackiness of the ink, and
by efficient metal catalysts which ensure the rapid through-curing of the printed ink film.
WO 03/066759 (and the related JP 2002-38065 and JP 01-289876) disclose a dual-curing ink matrix, comprising a UV curable material as the principal component (around 40 wt-%), together with an oxidatively curing alkyd resin as a secondary component (around 5 wt-%), a photoinitiator, and an oxidative polymerization catalyst. The disclosed ink composition does not comprise fusible wax.
This ink is subjected to UV curing immediately following the printing operation, whereupon it instantly dries, at least at the surface, with the consequence that set-off cannot occur. A slower, in-depth post-curing takes place during the following hours and days according to an oxypolymerization mechanism, allowing for a good adhesion of the ink to the substrate even in the presence of UV-opaque pigments or fillers.
The ink according to WO 03/066759 requires particular, e.g. EPDM rubber equipped, printing presses, designed for the printing of UV curing inks; the ink cannot be printed on an Intaglio printing press equipped for printing standard oxypolymerization curing, greasy inks.
WO 01/38445 A1 addressed the “set-off” of intaglio printing inks on polymer substrates. The binder of the therein disclosed intaglio printing ink includes an auto-oxidizable polyester resin having fatty acid residues, and a wax dispersion having a glass transition temperature below the maximum temperature achieved during the printing process. The disclosed printing ink further includes solvents and pigments and can be cured under UV radiation. This printing ink contains no acrylates at all.
The majority of intaglio printing inks used today are still alkyd based, greasy inks, which cure according to a purely oxidative drying mechanism. They traditionally contain hydrocarbon solvents. In consequence, the printing machines in the majority of printing works are equipped with inking systems, printing blankets and wiping cylinders which are specifically designed to resist to the alkyd- and hydrocarbon solvent-based chemistry of these traditional intaglio printing inks, but which, in turn, do not resist to the more polar UV-ink chemistry.
Oxidatively drying alkyds, as compared to UV-curing inks have, however the shortcomings of an inherently slow drying, which results in a lower production rate, of the need to use environment-unfriendly organic solvents (VOC=volatile organic compounds), and of the intrinsic proneness of these inks to produce “set-off” as a consequence of their slow drying. Their main advantage, in turn, is a good in-depth curing provided by the oxidative drying mechanism, resulting in good physical and chemical resistances of the printed and dried product. The printing equipment adapted to print them is furthermore already in place at every printing work.
UV-curing intaglio printing inks, on the other hand, have the advantage of a fast or almost immediate surface drying, eliminating waiting times and allowing for a high production rate. The presence, in the ink formulation, of volatile organic compounds can be avoided, and set-off does not occur due to the instant-drying.
The shortcomings of UV-inks, in turn, are that in-depth curing remains a challenge, in particular in case of a high pigment loading in the ink and/or the presence of pigments which are opaque or which have a high absorbance in the UV spectrum. UV-curable intaglio printing inks are furthermore significantly more expensive than traditional alkyd based inks, and, even more important, the printing equipment needs a major change of all components which come into contact with the UV-curable printing ink, in particular the rollers made of rubber or other polymer materials, which must be redesigned to resist the different chemistry of the UV-inks.
The chemical composition of UV-curing intaglio printing inks is noteworthy entirely different from that of alkyd-/hydrocarbon solvent based intaglio printing inks. When UV-curable intaglio printing inks come in contact with the alkyd-/hydrocarbon solvent-specific rubber components of the inking system, the printing blankets and the wiping cylinders of the printing machine, they can cause a swelling or shrinking of the rubber, which in turn alters the geometry of the rollers and blankets. This results in a low printing quality, as well as in a reduced roller lifetime, altogether increasing the printing and maintenance cost.
In practice, to allow for the printing of UV-curing intaglio inks, the rollers of the printing machine must be made of a special material or protected by a highly resistant compound such as non-polar EPDM rubber (ethylene propylene diene monomer rubber). Thus an additional cost arises for the printer if he changes from traditional alkyd-based intaglio inks to energy-curable intaglio inks, which is caused on the one hand by the more expensive energy-curable (UV-curable) intaglio printing ink itself, and on the other hand by the expensive upgrade of the printing equipment to become UV-ink compliant. A further disadvantage results for the printer who needs to print in both technologies, because each time he changes the type of printing ink (UV-curable or oxidatively curable, respectively), all corresponding parts of the printing machine must be changed accordingly in a time-consuming operation.
It would thus be highly desirable to have available an ink which combines the favorable set-off properties of the UV intaglio inks with the good in-depth curing of the alkyd intaglio inks, which results in high physical and chemical resistances of the printed ink on the document, and which is compatible with (i.e. printable without change on) the existing intaglio printing equipment in place at the printers' premises.
It is the object of the present invention to provide an intaglio printing ink which has very good set-off resistance and in-depth curing values, and which can be printed on the conventional intaglio printing equipment designed for oxidatively curing inks.